Telecommunications systems typically using rings of optical fibers, where each optical fiber is able to carry a number of digital signals at different optical wavelengths. These fibers are connected to multiplexing and demultiplexing equipment that combine and separate the optical wavelengths. These fibers are interconnected into networks. Typically these networks have been arranged in rings, each fiber carrying approximately forty wavelengths.
A method for optical wavelength multiplexing and demultiplexing to provide free-space collimated optical beams at separate wavelengths that interface directly with a free-space optical switch is disclosed in Patel and Silverberg “Liquid Crystal and Grating-Based Multiple-Wavelength Cross-Connect Switch”, IEEE Photonics Technology Letters, Vol. 7, pp. 514–516, 1995(hereinafter “Patel”), using grating dispersion to separate the optical beams from two input and two output fibers. The number of optical input and outputs ports can be increased over the wavelength dispersive switch method discussed in Patel using an optical switch comprised of a two-dimensional array of micromirrors between two gratings discussed in U.S. Pat. No. 6,097,859 of Solgaard et al. One problem with such an implementation is that it is difficult to fabricate mirror arrays with perfect yield, leading to blocking network operation due to any defective mirrors in the mirror array. Wavelength independent input and output ports of the wavelength selective switch also are needed, in order to provide the ability to add and drop arbitrary wavelengths as required in mesh telecommunications networks.
Another optical switch discussed in U.S. Pat. No. 6,549,699 of Belser (hereinafter “Belser”) provides the ability to add and drop fiber ports using a single mirror to select the add-port fiber, which also determines the drop-port fiber and, therefore, does not allow for independent selection of add and drop ports. Such a switching configuration may not be useful in existing mesh telecommunications networks. Moreover, the switching configuration of Belser may only be scaleable to a few add-ports and drop-ports. The more add-ports and drop-ports that are required by a network, the larger the spacing between the mirror and the grating of Belser, which leads to mechanical drift over temperature. As a result, the optical switch discussed in Belser may not operate with large numbers of add and drop ports over a need temperature range.